Baltimore offensive tackle Michael Oher (74) blocks Denver’s Derek Wolfe during the season opener at Sports Authority Field at Mile High on September 5, 2013 in Denver Colorado. (Doug Pensinger/Getty Images)

When Joe Flacco takes his first snap Sunday, he will line up behind a left tackle who 10 weeks ago was toiling away in Jacksonville, a left guard who has already made as many stops as an Amtrak train, and a center who two years ago was blocking small-school defensive tackles.

The Ravens are starting just two of the offensive linemen who protected Flacco so well during their run to the Super Bowl last winter. One retired in the offseason. Another was traded in October. And one was placed on injured reserve last month.

This year has turned into an extreme chemistry experiment, but turnover on the line is not unusual for the Ravens or their peers.

“It’s the challenge that every team faces. It’s kind of the nature of the National Football League right now,” Ravens coach John Harbaugh said. “The game is won in the trenches, and the offensive line is the part that is the most complicated. … It’s hard to develop that chemistry and cohesion.”

The only constant in the NFL is change, especially up front, where free agency and the salary cap have made it difficult for teams to keep five linemen together from one season to the next, while limits on practice time and offseason workouts make it tough to get the new members up to speed.

The Ravens have reshuffled their line before each of Flacco’s six seasons. They have started 22 different linemen — from first-round draft picks to journeymen — since Harbaugh was hired in 2008. But this year’s group has been slower to gel. As a result, Flacco has been sacked 39 times and the Ravens are averaging a league-worst 2.9 yards per carry.

While there is more than one way to build an offensive line, and draft position and cap constraints may shape those philosophies, the consensus is that the biggest requirement is patience.

“I don’t think that there is any hard and fast formula because it depends on the guys in the group,” said former NFL lineman Ross Tucker, now an analyst for SiriusXM Radio. “Some groups might take five years. Other guys it might take five months. The only thing I do know for sure is the more time a group has together, the better off you’re going to be. Period.”

The Ravens’ philosophy in recent years has been to draft and develop offensive linemen instead of buying them on the open market or paying big bucks to keep them — an exception being Pro Bowl right guard Marshal Yanda. They have spent nine draft picks on linemen since 2008, including two in each of the past two drafts.

But because the Ravens have made the playoffs in five straight seasons and subsequently have drafted late in the first round every year, they have been unable to select elite prospects, particularly at left tackle, as their opponent on Sunday, the Minnesota Vikings, did when they selected Matt Kalil fourth overall after finishing 3-13 in 2011.

The Ravens have used just one first-round draft pick on a lineman since 2008: right tackle Michael Oher in 2009. Instead, they have targeted later-round prospects, some of them from smaller schools such as Delaware and Colorado State-Pueblo.

After a year sitting behind Matt Birk, who retired after the Super Bowl, Gino Gradkowski, a fourth-round pick out of Delaware, has stepped in as the starting center and has played better in recent weeks. But others, such as Ramon Harewood and Oniel Cousins, have not panned out.

“I’m a big believer that you can draft middle- and late-round picks and even have undrafted free agents on the practice squad for a year or two,” Tucker said. “And those guys learn your system, learn your techniques. … So after a year or two, they’re really ready to rock and roll.”

While teams such as the Houston Texans, New Orleans Saints and San Diego Chargers have constructed quality offensive lines mostly with later picks and cheaper free agents, other fearsome fivesomes from the Cincinnati Bengals, San Francisco 49ers and Denver Broncos have been built with at least three first- or second-round picks.

The Pittsburgh Steelers also used that approach, drafting center Maurkice Pouncey and right guard David DeCastro in the first round and offensive tackles Marcus Gilbert and Mike Adams in the second. But due to injuries and inexperience, that group has struggled.

The St. Louis Rams, with Jake Long and Scott Wells, have a rare effective line built through free agency.

Tucker believes that continuity and chemistry are just as important as ability when it comes to offensive line play.

“When you look at some of the groups that the (New York) Giants have had recently and those great lines the Broncos had (in the late 90s), they didn’t have any one real stud. But as a group, they perform the best,” Tucker said.

In recent years, the Ravens have been able to get into sync in time for the playoffs despite changes on the offensive line. In 2009, they drafted Oher and signed Birk. In 2010, they were forced to make changes after Jared Gaither hurt his back. In 2011, they added Bryant McKinnie. In 2012, they spent the regular season trying to fill the void left by Ben Grubbs.

It was much different a decade ago, when Brian Billick’s Ravens had mainstays on the offensive line in Jonathan Ogden, Edwin Mulitalo, Mike Flynn and Bennie Anderson.

The Ravens offensive line was mostly settled heading into this season. But they traded for left tackle Eugene Monroe and shipped the struggling McKinnie to Miami. When Kelechi Osemele was lost to a back injury, they were forced to start journeyman A.Q. Shipley at left guard.

“Our job is to get them in shape, get them going and get ourselves in position to win games,” offensive coordinator Jim Caldwell said two weeks ago. “I think we all understand that. That’s the bottom line. We have to find a way to get it done.”

Both Harbaugh and Caldwell said that the offensive line and the secondary are the two position groups that depend most on chemistry and cohesion. That doesn’t develop overnight, but it’s not for a lack of trying by the Ravens linemen, who spent all day in the practice facility together and chow down on the road together the night before games.

“We meet probably more than anybody,” Monroe said. “Having familiarity with the guys you’re working with is crucial to be successful long-term because so many different things happen on the field that require you to know the type of guy you’re playing next to.”

More change is expected this offseason. Monroe and Oher are both free agents. There could be another competition at center. And there are no guarantees Osemele will return to form after back surgery.

But with four games left and a playoff berth within their control, the Ravens aren’t looking that far ahead. The offensive line has played better the past two weeks, particularly when it comes to pass protection, and last year is proof that it is never too late for them to click.

“They’re starting to really come together,” Flacco said. “The reason you win football games in this league is because of those guys. So a lot of credit should go to those guys for getting these last couple wins. And a lot of credit is going to need to go to these guys the rest of the way out when we go play the way we’re capable of playing of.”

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